The first time Rep. Tom Tancredo got really angry about immigration, the year was 1975, and he was a junior high school social studies teacher in Denver. The state had recently passed the nation's first bilingual education law, and Hispanic kids were taken from his class to study in Spanish.

That idea made zero sense to Tancredo, the grandson of Italian immigrants. He believed that newcomers should be assimilated into the country, as they had been for generations. The image of America as a beacon for people from all over the world uniting under one flag and one language was threatened, he contended, if the country started adapting to immigrants, instead of the other way around.

A year later, Tancredo launched a political career animated by his obsession to stem the tide of immigration from Mexico and Central America that he feared would change the character and security of the country. Today, the four-term Republican House member stands at the center of a national debate over how best to deal with the nearly 12 million illegal immigrants in the United States. Tancredo helped the House pass a bill in December that would impose criminal sanctions on illegal immigrants and those who employ them and that would erect a wall along 700 miles of the U.S.-Mexican border to keep others out.

That legislation has triggered massive protests throughout the country and prompted a Senate committee this week to pass an alternative measure with a guest-worker program that would help many illegal immigrants eventually win permanent residency or even U.S. citizenship.

Tancredo, 60, has so effectively tapped into the anger of millions of Americans who favor a crackdown on illegal immigrants and tougher measures at the border that the back-bench Republican is considering making a bid for president in two years. But in Washington, he is viewed warily by Democrats, the White House and even some of his Republican colleagues as a loose cannon or even a zealot.

After Tancredo suggested in 2002 that President Bush's views on immigration amounted to a national security threat, White House political adviser Karl Rove told him, "Don't ever darken the door of the White House" -- although Tancredo later was invited to the White House for a bill signing.

Democrats have criticized him for stoking anti-immigration sentiment, although some see his tactics as helping them by driving Hispanic American voters in Florida, California and other states away from the Republican Party. "I'm all for more and more nuts in their party speaking up," said Rep. Rahm Emanuel (D-Ill.), who chairs the House Democrats' congressional campaign committee. "I want more of those guys."

Tancredo shrugs off the criticism. "It's not a secret that I have burned a lot of bridges here," he said in an interview this week. "My ability to get anything done around here is based around my ability to make this into a national issue. My megaphone is pointed at the ear of America."

A ubiquitous presence on the airwaves, Tancredo has appeared more than 1,000 times on radio talk shows in recent years and has become a television news mainstay. He has traveled widely around the country, including the early presidential caucus and primary states of Iowa and New Hampshire. His office averaged 4,566 pieces of mail per month during the first quarter of this year. When Tancredo was scheduled as a guest on ABC's "This Week" last Sunday, he received 300 "good luck" e-mail messages before his appearance and 700 "good job" e-mails after the show.

The relentless media exposure, along with polls indicating that many Americans support a border crackdown and the deportation of illegal immigrants, has given Tancredo considerable influence over the immigration legislation now unfolding in Congress. "He's a force because he represents what a lot of people think," said Steven Camarota of the Center for Immigration Studies, which favors less immigration. "He's only a gadfly in Washington."

For many frustrated, scared and angry suburbanites and small-town residents, Tancredo is a hero, one of the very few Washington politicians who take their fears seriously. They believe undocumented, mostly Hispanic workers are taking jobs that ought to go to citizens, flooding schools and boosting the crime rate, and that the country's open borders pose a security threat.

"My parents were immigrants," says Mark Solinsky, an independent voter from the sprawling Denver suburb of Highlands Ranch, a part of Tancredo's suburban Colorado district. "Came from Poland. In the '20s. But I think now, this new wave of immigration poses a problem. We've got to have tougher laws. And, you know, that's what Tancredo always talks about. So I would say, yeah, he's right on that. We need some leadership on that."